Here are some diagrams of open and closed cooling systems. Open systems allow coolant to evaporate, causing the need to add coolant more frequently.
kuerbis
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DUKEHag
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DribbleDuke
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kuerbis
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TC's SD
I agree with Kuerbis, i havent added coolant in 5k miles, which would lead me to believe the system is closed...? Of course I'm not sure what difference this would make with regard to coolant anyway?